Electrochemical process can control a reaction by changing electric current or voltage, and enables oxidation reaction and reduction reaction to conduct at separate place. Thus, the electrochemical process is a basic technology widely used in electrolytic soda process, electroplating, metal collection or the like.
It is known that when an aqueous solution is electrolyzed, oxidizing agents such as oxygen, chlorine, ozone, hydrogen peroxide, or the like can generate, and active species such as OH radicals also partially generate. In water treatment field such as cleaning, sterilization or waste water treatment, electrochemical process utilizing those oxidizing agents is getting to be employed. In particular, in water treatment field, the electrochemical process is not always required to use medicines, and is therefore noted as a clean technology that can achieve the objective treatment with only electric energy.
Thus, the electrochemical process is put to practical use. However, life of an electrode is not sufficient depending on the purpose of use, and this disturbs spread of the process. It is also pointed out that depending on the purpose of use, the objective reaction does not proceed sufficiently. When an aqueous solution is electrolyzed, discharge reaction of water easily proceeds, and the objective reaction proceeds slowly, resulting in decrease in energy efficiency. Cases have frequently occurred that electrochemical reaction cannot be applied from the economic standpoint.
Conventional electrodes for electrolysis include lead oxide, tin oxide, platinum, DSE (Dimensionally Stable Electrode), graphite and amorphous carbon. Graphite or amorphous carbon material has conventionally been used as an electrode material. However, those are consumable materials, and show remarkable consumption in anodic polarization. It is known that lead oxide and tin oxide also consume in anodic polarization.
Platinum and DSE are called an insoluble electrode. Valve metal such as titanium, having high corrosion resistance at anodic polarization, and its alloy are used as an electrode substrate of such an insoluble electrode. Noble metals such as platinum or iridium, having high corrosion resistance, and their oxides are used as an electrode catalyst. However, it is known that even where those expensive corrosion-resistant materials are used, a substrate corrodes according to current density and current-carrying time when electric current is applied, and an electrode catalyst gradually consumes. Thus, an electrode having further excellent durability is demanded.
Diamond has excellent thermal conductivity, optical transmission and durability against high temperature and oxidation, and in particular can control electric conductivity by doping. Therefore, diamond is considered to have great potential as semiconductor device or energy conversion element.
Swain et al. reported that conductive diamond has stability as an electrode catalyst for electrolysis in an acidic solution, and suggested that diamond is far superior to other carbon material (Journal of Electrochemical Society, Vol. 141, 3382 (1994)). JP-A-7-299467 proposes a treatment method of organic substances in a waste liquid by oxidation decomposition using a conductive diamond electrode having conductive diamond as an electrode catalyst. JP-A-2000-226682 proposes a method of electrochemically treating organic substances, in which conductive diamond is used as an anode and also a cathode. JP-A-11-269685 proposes an electrochemical synthesis method of ozone using a conductive diamond electrode as an anode. JP-A-2001-192874 proposes synthesis of peroxosulfuric acid using a conductive diamond electrode as an anode.
From such researches, a conductive diamond is noted as an electrode catalyst from the standpoints of corrosion resistance and efficiency. It is expected that when an electrochemical process using a conductive diamond electrode is employed, decomposition efficiency of organic substances or synthesis efficiency of useful oxides such as peroxosulfuric acid is improved, as compared with the case of using the conventional electrode.
To use a conductive diamond as an electrode for electrolysis, a substrate to maintain a structure as an electrode and also to supply sufficient electric current is required. Therefore, it is necessary to constitute an electrode by depositing a conductive diamond on such a substrate. Hot filament CVD (Chemical Vapor Deposition) method, microwave plasma CVD method, plasma arc jet method, PVD (Physical Vapor Deposition) method and the like are developed as a synthesis method of conductive diamond. In CVD synthesis method which is the general production process of a conductive diamond, an electrode is exposed to hydrogen atmosphere at high temperature of 750-950° C. For this reason, it is essential or desirable that the electrode substrate is thermally and chemically stable, is difficult to undergo hydrogen brittleness, and has a coefficient of thermal expansion close to that of diamond. Non-metal materials such as silicon, silicon carbide, graphite or amorphous carbon, and metal materials such as titanium, niobium, zirconium, tantalum, molybdenum or tungsten are reported as a substrate for a conductive diamond electrode satisfying those requirements. Silicon or niobium is put into practical use from the standpoint of corrosion resistance.
Recently, with progress of applied researches of a conductive diamond into various electrochemical processes, it has been revealed that even a conductive diamond electrode having a substrate comprising silicon or niobium does not have industrially sufficient durability depending on the applied uses. As a result of investigations of this cause, it has been confirmed that an electrolytic solution impregnates into defective portions such as pinholes or cracks, present in a diamond catalyst layer, a substrate corrodes, and peeling of the diamond catalyst layer proceeds with the corrosion. Those defective portions unavoidably generate by scattering or ununiformity of a diamond synthesis step to corrode a substrate, thereby shortening a life of electrode. For this reason, a substrate overcoming those disadvantages and having further improved corrosion resistance is demanded.
As described above, it is considered that an electrolytic solution impregnates into a diamond catalyst layer from defective portions such as pinholes, unavoidably generated therein, a substrate corrodes, and as a result, peeling of the diamond catalyst layer proceeds. From this standpoint, it is essential to prevent corrosion of a substrate in order to provide a stable electrode.
A method of forming an oxide layer on a surface of a substrate for the purpose of improving adhesion between an electrode catalyst and a substrate and also protecting a substrate itself is disclosed as a basic life-prolonging method of DSE in an oxide electrolytic cell (JP-A-57-192281). However, even where such an oxide interlayer is formed on the surface of a substrate for a conductive diamond electrode, the most part of the oxide interlayer is reduced by hydrogen radicals or the like under CVD diamond synthesis conditions. As a result, in many cases, the objective functions of adhesion improvement of the electrode catalyst and corrosion resistance improvement of the substrate are not exhibited.
Conventionally, rapid voltage rise called anode effect is frequently observed in electrolysis of a molten salt containing a fluorine compound, using a carbonaceous material Such as graphite or amorphous carbon as an anode. It is confirmed that the cause of this phenomenon is that fluorinated graphite is formed on the surface of an electrode, thereby inhibiting wettability between an electrode and an electrolytic solution (Fluorine Chemistry and Industry, Kagaku Kogyo Sha).
Regarding influence of a conductive diamond to a diamond catalyst layer by fluorination treatment, Sine et al. confirm that when plasma fluorination treatment is applied to a conductive diamond electrode having a substrate comprising monocrystal silicon, the diamond catalyst layer is fluorinated, and report that regarding its electrochemical properties, oxygen generation overvoltage at anodic polarization is the same as that of the conductive diamond electrode before fluorination treatment, but hydrogen generation overvoltage at cathodic polarization increases (Electrochemical and Solid-State Letters, 6(9)D9-D11 (2003)).
Electrochemical properties of the conductive diamond electrode vary by fluorination treatment, but the electrochemical function thereof is not impaired. Ferro et al. suggest the possibility that a life at anodic polarization of a fluorinated conductive diamond catalyst obtained by plasma fluorination treatment of a monocrystal silicon substrate-bearing conductive diamond electrode is prolonged (Journal of Physical Chemistry B 2003, 107, 7567-7573).